Add queue deduplication to prevent duplicate entries

- In-memory dedup in add_to_queue() using _pending_by_episode dict
- Batch-local dedup via seen_in_batch set (handles duplicates within single call)
- Database unique index on episode_id via __table_args__
- 5-minute cooldown in _auto_download_missing() to prevent rapid re-triggers
- Updated _add_to_pending_queue() and _remove_from_pending_queue() to track episode keys
- Added TestQueueDeduplication with 4 test cases
- Updated DEVELOPMENT.md and TESTING.md with queue dedup docs

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/unit/test_download_service.py

@@ -834,3 +834,111 @@ class TestRemoveEpisodeFromMissingList:
         # Episode 2 should be removed from in-memory missing list
         assert 2 not in serie.episodeDict[1]
         assert serie.episodeDict[1] == [1, 3]
+
+
+class TestQueueDeduplication:
+    """Test queue deduplication to prevent duplicate entries."""
+
+    @pytest.mark.asyncio
+    async def test_add_same_episode_twice_creates_only_one_entry(
+        self, download_service
+    ):
+        """Test that adding the same episode twice only creates one queue entry."""
+        episodes = [EpisodeIdentifier(season=1, episode=1)]
+
+        # Add same episode twice
+        ids1 = await download_service.add_to_queue(
+            serie_id="series-1",
+            serie_folder="series",
+            serie_name="Test Series",
+            episodes=episodes,
+        )
+        ids2 = await download_service.add_to_queue(
+            serie_id="series-1",
+            serie_folder="series",
+            serie_name="Test Series",
+            episodes=episodes,
+        )
+
+        # Should only have one entry
+        assert len(download_service._pending_queue) == 1
+        # First call creates one ID
+        assert len(ids1) == 1
+        # Second call creates zero IDs (deduplicated)
+        assert len(ids2) == 0
+
+    @pytest.mark.asyncio
+    async def test_add_different_episodes_creates_separate_entries(
+        self, download_service
+    ):
+        """Test that different episodes create separate queue entries."""
+        episodes1 = [EpisodeIdentifier(season=1, episode=1)]
+        episodes2 = [EpisodeIdentifier(season=1, episode=2)]
+
+        ids1 = await download_service.add_to_queue(
+            serie_id="series-1",
+            serie_folder="series",
+            serie_name="Test Series",
+            episodes=episodes1,
+        )
+        ids2 = await download_service.add_to_queue(
+            serie_id="series-1",
+            serie_folder="series",
+            serie_name="Test Series",
+            episodes=episodes2,
+        )
+
+        # Should have two separate entries
+        assert len(download_service._pending_queue) == 2
+        assert len(ids1) == 1
+        assert len(ids2) == 1
+        # IDs should be different
+        assert ids1[0] != ids2[0]
+
+    @pytest.mark.asyncio
+    async def test_add_same_episode_different_series_creates_entries(
+        self, download_service
+    ):
+        """Test that same episode in different series creates separate entries."""
+        episodes = [EpisodeIdentifier(season=1, episode=1)]
+
+        ids1 = await download_service.add_to_queue(
+            serie_id="series-1",
+            serie_folder="series1",
+            serie_name="Test Series 1",
+            episodes=episodes,
+        )
+        ids2 = await download_service.add_to_queue(
+            serie_id="series-2",
+            serie_folder="series2",
+            serie_name="Test Series 2",
+            episodes=episodes,
+        )
+
+        # Should have two separate entries (different series)
+        assert len(download_service._pending_queue) == 2
+        assert len(ids1) == 1
+        assert len(ids2) == 1
+
+    @pytest.mark.asyncio
+    async def test_add_multiple_episodes_with_duplicates_filters_correctly(
+        self, download_service
+    ):
+        """Test that adding multiple episodes with some duplicates filters correctly."""
+        episodes = [
+            EpisodeIdentifier(season=1, episode=1),
+            EpisodeIdentifier(season=1, episode=2),
+            EpisodeIdentifier(season=1, episode=1),  # duplicate
+            EpisodeIdentifier(season=1, episode=3),
+        ]
+
+        ids1 = await download_service.add_to_queue(
+            serie_id="series-1",
+            serie_folder="series",
+            serie_name="Test Series",
+            episodes=episodes,
+        )
+
+        # Should only have 3 entries (1, 2, 3) - one filtered out
+        assert len(download_service._pending_queue) == 3
+        assert len(ids1) == 3